1. Field of the Invention
The present invention relates to a drawing apparatus consisting of a drawing die assembly and a double action mechanical press of the type having a fixed blank holder table, an upper die slide and a lower punch slide (herein referred to as a lower punch slide type double action press).
2. Description of the Prior Art
A double action press is suited for performing a drawing process of a pressed part as is well known in the art. However, when an ordinary double action press is utilized in combination with a transfer press for the following processes of the pressed part, it may cause a problem that the part drawn thereby needs to be turned over before fed to the transfer press. In that case, the aforementioned lower punch slide type double action press is utilized to eliminate the turn-over process.
FIGS. 1 and 2 show a prior art drawing apparatus 10 of the double action and lower punch slide type. The drawing aparatus 10 consists of a drawing die assembly 12 and a lower punch slide type double action press 14. The lower punch slide type double action press 14 includes a fixed blank holder table or bolster 16 mounted on a bed 18, an upper die slide 20 and a lower punch slide 22. The blank holder table 16 is formed with a stepped hole 16a for allowing the lower punch slide 22 to come thereinto upon an upward stroke of same. The stepped hole 16a has a shoulder 16b. The drawing die assembly 12 includes a blank holder 24 fixedly mounted on the table 16 by way of a blank holder plate 26, a die 28 mounted on the upper slide 20 and a punch 30 movably guided by the blank holder 24 and fixedly attached to a punch plate 32 which is in turn supported on the shoulder 16b of the blank holder table 16. Indicated by the reference numeral 34 are a plurality of shims placed between the punch plate 32 and the shoulder 16b for the adjustment of the bottom dead position of the punch 30 (i.e. the position which the punch 30 assumes when disengaged from the lower punch slide 22).
In operation, a blank W is first placed on the blank holder 24. The upper die slide 20 is then caused to come down to a position where the blank W is clamped between the die 28 and the blank holder 24. The upper die slide 20 remains at that position for a while during which the lower punch slide 22 having come in contact with the punch plate 32 further moves upward to cause the punch 30 to protrude into the die 28 for thereby drawing the blank W into a predetermined shape as shown in FIG. 2.
In the foregoing prior art drawing apparatus, the shims 34 are in effect indispensable for making the upper end of the punch 30 nearly flush with the upper end surface of the blank holder 24 so that the blank W is held not only by the blank holder 24 but by the upper end of the punch 30 when supplied to the drawing die assembly 14. If the upper end of the punch 30 is not flush with, e.g. considerably lower than the upper end surface of the blank holder 24, such a case may occur in which the blank W is curved too much to be subject to a proper drawing operation. This is particularly true when the blank W is considerably large relative to its thickness and the article to be drawn from the blank W is relatively shallow. The setting of the shims 34 however is quite laborious since adjustement of the shims 34 requires removal of the punch 30 and the blank holder 24. Furthermore, a different drawing die assembly requires a different set of shims, making the die setting operation more laborious.